mypy ignores possible changes of global variables outside current function

[lilydjwg]

Bug Report

See code.

To Reproduce

from typing import Optional

class Global:
  attr: Optional[int] = None

G = Global()

def f() -> None:
  G.attr = 1

def t() -> None:
  G.attr = None
  f()
  assert G.attr is not None
  reveal_type(G)

1. Write the above code
2. Run mypy on it

Expected Behavior

Mypy shows the type of G at the last line (and G.attr is of type int).

Actual Behavior

Mypy completely skips the lines below assert.

Success: no issues found in 1 source file

Your Environment

• Mypy version used: 0.910
• Mypy command-line flags: no other args than the file
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.9.6
• Operating system and version: Arch Linux current

[KotlinIsland]

#11649
#11969

This issue a well known limitation.